Cleta Mitchell, one of Washington's most respected elections attorneys, told Newsmax she has tangible proof that high-ranking IRS officials in Washington were fully aware of the agency's campaign to target conservative groups for heightened scrutiny, despite their denials.

And she thinks the president knew about the practice, too. If proven, she said, it could be an impeachable offense.

Mitchell, in an interview with Newsmax TV on Tuesday, said she was told by a Cincinnati IRS agent that applications by two of her conservative clients were being processed by -- and would ultimately be approved or denied in -- Washington.

She said she also is aware of nearly 100 other conservative groups that were being targeted by Washington.

"There were nearly 100 groups across the country that got the very egregious set of letters from the IRS that were almost identical and they came from offices all over the country so I know of at least 85 to 90, maybe more, organizations," said Mitchell, who represents six groups which say they have been targeted, including the King Street Patriots and True the Vote.

"If they had the name 'tea party' or they had the word 'patriots' or if their mission was smaller government or study the Constitution, believe it or not, that would cause the IRS to say, 'Oh, we better investigate these groups.'"

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She added she had two clients whose group's purpose was to lobby against Obamacare, both of which received extra IRS scrutiny. And of the clients who have gone public with their claims, they received "incredible scrutiny, voluminous requests for information, documents, almost like having been audited before they even are an exempt organization."

In the case of one such client, she and her family subsequently became targets for audits to their personal and business tax returns, and were even visited by three different government agencies. She also knows of other groups who had surprise visits from the FBI after they applied for IRS status.

Mitchell said she doesn't believe the president or the White House was uninvolved in the IRS activities, as the administration has claimed.

"I've thought for some time that this is politically motivated and that's the reason it was happening. And, as I said, I've been doing this for more than 20 years and I've never seen anything like this until 2009, 2010. And the only thing that changed was we had a different administration," she said.

"We know the White House used the Department of Health and Human Services to try to silence critics about Obamacare. So if we know they used HHS, why wouldn't they also use the IRS or other federal agencies to try to silence political critics?"

Mitchell credits Congress for investigating the matter but says they have had limited effectiveness because she believes the IRS has lied even to lawmakers during hearings last year.

"They've been very helpful but the fact is, the IRS has lied and covered up, even to the members of Congress," she said. "The problem is they've had hearings -- the IRS commissioner basically lied to Congress last year when he appeared before Congress and they asked him about this targeting conservative groups and he said it wasn't true. Well, no, we find out yes, it was true."

She added, "They may try to say it was low-level people. It was not low-level people. They weren't in Cincinnati. It was being directed out of Washington, and I have them on record saying that."

She said it's "a criminal offense to misuse information submitted by a taxpayer or an entity, anybody who submits anything to the IRS. The IRS agents are limited in what they can do with it, the scope of what they can say and do with it. So, clearly, the federal law has been broken."

Asked whether it would be an impeachable offense if it emerged that the president or his officials were behind the IRS' practices, Mitchell said, "Well, it certainly was for Richard Nixon 40 years ago this week."

She added, "Isn't that ironic? The House of Representatives passed articles of impeachment against President Richard Nixon 40 years ago this week on May 18, and one of those was misuse of the IRS to go after political enemies."

Cleta Mitchell, one of Washington's most respected elections attorneys, told Newsmax she has tangible proof that high-ranking IRS officials in Washington were fully aware of the agency's campaign to target conservative groups for heightened scrutiny, despite their denials.